The invention relates to a method for identifying deviations in quantity in the case of a fluidic or hydraulic, respectively, metering system, in particular of an internal combustion engine of a motor vehicle, in which at least one conveying pump for conveying a fluid, and at least one pressure sensor for determining a pressure in the metering system, are disposed. The invention furthermore relates to a computer program, to a machine-readable data carrier for storing the computer program, and to an electronic control apparatus by means of which the method according to the invention is capable of being carried out.
In order for the pollutant emissions of motor vehicles to be restricted, the permissible limit values are being increasingly lowered worldwide. Presently, amended emission legislation (for example Euro 6) and new test cycles (WLTP: Worldwide Harmonized Light Duty Test Procedure or RDE: Real Driving Emission, expected as from 2017) will further reduce said permissible values. Adhering to these limit values in all vehicle classes will only be possible by way of an active exhaust gas post-treatment.
The Denoxtronic (DNOX) of an SCR (Selective Catalytic Reduction) metering system serves for injecting so-called “AdBlue”, a urea/water solution (UWS) of 32.5% urea in water, into the exhaust gas flow ahead of an SCR catalytic converter. The urea is converted to ammonia by thermal decomposition and hydrolysis. The nitrogen oxides in the SCR catalytic converter are reduced to water and nitrogen by the ammonia.
In modern diesel engines having an SCR system, NOx raw emissions are substantially reduced by means of a so-called DNOX urea metering system. Future DNOX systems comprise a volumetric conveying pump or conveying module, respectively, and a pressure sensor. The systems are operated in a purely controlled manner and have a return flow to the tank mentioned for any non-consumed AdBlue solution. The conveying pump in most instances represents a displacement pump having an analogue construction principle or an analogue operating mode, respectively. The (conveying pump) mass flow can thus be determined in a relatively simple manner if the displacement volume is known.
A method for monitoring the quantity of a metering system concerned here is derived from DE 10 2013 218 897 A1, in which deviations of the conveying output of a conveying pump mentioned, and any potential leakage of the line system, are identified. The accuracy of the monitoring of the quantity here is substantially determined by the variable mechanical or fluidic, respectively, rigidity of the line system. The rigidity also depends on the ageing and the production tolerances of the system components used, the influence thereof on the rigidity not being known or foreseeable, respectively, or being able to be determined only by way of a relatively high effort.
A method for diagnosing a metering valve is also derived from DE 10 2013 207 867 B4, in which a pressure drop in a reagent that arises during a diagnosis metering is evaluated by way of a comparison with a threshold value. An error in the metering valve is identified when the threshold value is exceeded. The pressure loss to be expected is determined in an experimental manner.